There was an article in the October 9 2010 issue of The Economist titled
which prompted me to consider the importance of non-replenished ground water on the atmospheric water vapor content when this deep water is transfered to the surface and evaporates. Non-replenished water is called “fossil water“.
This is an important climate issue which seems to have been overlooked. The Economist article includes the text
“….. aquifers are still poorly understood. Until a UNESCO inventory in 2008, nobody knew even how many transboundary aquifers existed. Experts are still refining the count: the American-Mexico border may include 8, 10, 18 or 20 aquifers, depending on how you measure them. Defining sustainability vexes hydrologists too, particularly with ancient fossil aquifers that will inevitably run dry eventually. Estimates for the life of the Nubian sandstone aquifer range from a century to a millennium.”
Fossil water as written in Wikipedia is
Fossil water or paleowater is groundwater that has remained sealed in an aquifer for a long period of time. Water can rest underground in “fossil aquifers” for thousands or even millions of years. When changes in the surrounding geology seal the aquifer off from further replenishing from precipitation, the water becomes trapped within, and is known as fossil water.
The Ogallala Aquifer and Nubian Sandstone Aquifer System are among the most notable of fossil water reserves. Fossil aquifers also exist in the Sahara, the Kalahari, and the Great Artesian Basin. A further potential store of ancient water is Lake Vostok, a subglacial lake in Antarctica.
Fossil water is a non-renewable resource. Whereas most aquifers are naturally replenished by infiltration of water from precipitation, fossil aquifers get very little recharge. The extraction of water from such non-replenishing groundwater reserves (known as low safe-yield reserves) is known in hydrology as “water mining”. If water is pumped from a well at a withdrawal rate that exceeds the natural recharge rate (which is very low or zero for a fossil aquifer), the water table drops, forming a depression in the water levels around the well. Water mining has been blamed for contributing to rising sea levels.
An important climate question is the contribution of this fossil water, through irrigation, to local, regional and global transpiration and evaportion from soils and other surfaces. This is a flux of water vapor into the atmosphere that would otherwise not occur.
Fossil water reservoirs can be quite large. For example, in the paper
Issar, A, 1985: Fossil Water under the Sinai-Negev Peninsula. Scientific American Vol. 253, No. 1, p 104-110, July, 1985
the abstract reads
“A study of water issuing from springs and wells scattered across the Sinai (Egypt) and the Negev (Israel) deserts has identified a great aquifer formed during the last glacial age. Satellite photographs of the region reveal surface characteristics consistent with subterranean geology that could support an aquifer. Carbon-14 dating puts the age of the water from springs and wells at 20,000 to 30,000 yr. The ages determined from C-14 dating agree with results of hydrological flow models. Water samples from the ‘ Ayun Musa, from the abandoned oil-exploration well dug into the Nubian sandstone layer in Nakhel and from the artesian wells in the Nubian sandstone layer near the Dead Sea all have the same relative amounts of deuterium and oxygen-18. The chemical and isotopic studies in conjunction with archaeological evidence suggest that the aquifer holds rainwater that was trapped during the most recent ice age. It has been calculated that the Nubian sandstone aquifer under the Sinai and the Negev holds 200 billion cu m of water, 70 billion cu m of which is under the Negev. Agricultural settlements in the Negev demonstrate that the water is low enough in salt content to be suitable for irrigation.”
Recently, there was a report of the Liyban government’s program to ultilize fossil water for irrigation;
It is reported in the above news article
The Libyan government says the 26-year project has cost $19.58 billion. Nearing completion, the Great Man-Made River is the largest irrigation project in the world and the government says it intends to use it to develop 160,000 hectares (395,000 acres) of farmland. It is also the cheapest available option to irrigate fields in the water-scarce country, which has an average annual rainfall of about one inch.
I have asked my colleague, Faisal Hossain at Tennessse Tech University ,who is an internationally well-respected hydrologist about this issue, and he replied that, based on the paper
Freydank, K. and Siebert, S. Towards mapping the extent of irrigation in the last century: a time series of irrigated area per country. Frankfurt Hydrology Paper 08, Institute of Physical Geography, University of Frankfurt, Frankfurt am Main, Germany, (2008).
that globally about 60% of irrigation water is from ground water, but the fraction that is from fossil water does not appear to have yet been assessed. Professor Hoassin reminded me that the paper
[UPDATED PM Oct 18 2010 to correct citation and with edits in the associated text]
DeAngelis, A., F. Dominguez, Y. Fan, A. Robock, M. D. Kustu, and D. Robinson (2010), Evidence of enhanced precipitation due to irrigation over the Great Plains of the United States, J. Geophys. Res., 115, D15115, doi:10.1029/2010JD013892.
claims that groundwater in the Midwest from the Ogallala Aquifer(much of which is fossil water) doubled its contribution in terms of evaporation in the 20th century.
There is another paper on the role of irrigation in the global climate. It is
Puma, M. J., and B. I. Cook (2010), Effects of irrigation on global climate during the 20th century, J. Geophys. Res., 115, D16120, doi:10.1029/2010JD014122
I posted on this paper in
Among the recommendations in the Puma et al (2010) paper is
“Future efforts to understand irrigation in a climate model setting should not only carefully document the amount of irrigation water applied to the land, but also keep track of the relative amounts of surface water and groundwater used for irrigation.”
Faisal also referred me to the paper
Scanlon, B. R., I. Jolly, M. Sophocleous, and L. Zhang (2007), Global impacts of conversions from natural to agricultural ecosystems on water resources: Quantity versus quality, Water Resour. Res., 43, W03437, doi:10.1029/2006WR005486
which includes the excerpts from the abstract
“Past land use changes have greatly impacted global water resources…… Since the 1950s, irrigated agriculture has expanded globally by 174%, accounting for ~90% of global freshwater consumption. ….. Long time lags (decades to centuries) between land use changes and system response (e.g., recharge, streamflow, and water quality), particularly in semiarid regions, mean that the full impact of land use changes has not been realized in many areas and remediation to reverse impacts will also take a long time…….”
We recommend an extension to these studies. Particularly, that ground water be further broken into replenishable ground water and fossil water, and an assessment of the local, regional and global contribution of fossil water used for irrigation to the transfer of water vapor into the atmosphere.
Fossil water, as with fossil fuels, involves the insertion of a climate forcing into the atmosphere due to human activities of a gas (in this case additional water vapor) which would otherwise not be there.